The present invention relates to a transmission system capable of transmitting modem signals or multifrequency (MF) signals together with speech signals over the same digital transmission line.
A coding system using adaptive differential pulse modulation (ADPCM) and a multi-pulse coding system are highly efficient implementations for coding speech signals, and are well known in the art. A problem encountered in the prior art is that when modem signals, or MF signals which are different from speech signals, are transmitted over the same digital transmission line as speech signals by such a high efficient coding system, the coded MF signals or modem signals cannot be decoded without errors. In the light of this, it has been customary to provide a digital transmission line which is adapted for a highly efficient coding system independently of a separate digital transmission line adapted for the transmission of modem signals or MF signals, at the sacrifice of cost.